1. Field of the Invention
This invention relates to a plasma display device used as various thin-type display panels, and more particularly to a plasma display panel in which a fluorescent substance is excited with energy of ultraviolet rays to produce visible light.
2. Description of Related Art
Plasma display panels (PDP) can be made dramatically small in depth compared with cathode-ray tube direct-view type display units and rear projection display units, and have been expected as a promising means for realizing wall type large-screen televisions in the future. At present, however, such plasma display panels are on such a level of development that they have still a lower contrast ratio and brightness than the existing display units. In order for them to come into wide use in the future, it is essential to achieve a great improvement in such performances.
Under such circumstances, as a measure for improving contrast ratios and color purities of plasma display panels, Japanese Patent Applications Laid-open (KOKAI) No. 59-36280 and No. 61-6151, for example, disclose techniques in which optical filters formed of an inorganic material are imparted to individual cells.
In these techniques, the optical filters are dividedly arranged so as to correspond to individual cells, on a glass substrate provided in front of a cell board, and have transmittance corresponding to the luminescent colors of the individual cells. Spectra of light emitted from fluorescent substances provided inside the individual cells change correspondingly to the transmittance of the filters to bring about an improvement in color purity of red, green and blue each.
The fluorescent substances used in plasma display panels commonly tend to reflect light coming from the outside (i.e., ambient light). Especially in an environment having bright surroundings, they may cause a rise in apparent black level to tend to cause a decrease in contrast ratio of display units. The optical filters provided correspondingly to the individual cells attenuate the ambient light incident on the fluorescent substances and also again attenuate the ambient light component reflected from the fluorescent substances before it is emergent outside, so that the contrast ratio in a bright environment can be greatly improved.
In the above conventional techniques, process temperatures in the production of plasma display panels are estimated to be about 500.degree. C. to about 600.degree. C., and hence inorganic materials resistant to high temperatures are used in the optical filters. If, however, the process temperatures can be dropped to about 250.degree. C., it is possible to use optical filters made of organic materials that enable much sharper change in transmittance and is possible to more improve color purity.
The above filter technique is supposed to step by step bring about improvements of color purities of the three primary colors, red, green and blue. In the case of plasma display panels, however, luminescent color of discharge gas sealed in panels is a great factor that obstructs the improvement in color purity. As the discharge gas sealed in panels, a gas chiefly composed of neon (Ne) gas and mixed with xenon (Xe) gas, helium (He) gas or argon (Ar) gas is usually in wide use taking account of discharge efficiency. The neon gas has an emission spectrum, as shown in FIG. 7, formed of a combination of several peak wavelength components distributed to range from the latter half of 500 nm to 700 nm, among which a component having the greatest energy is the component at 585 nm. Hence, the neon gas is discharged in orange color, which is commonly called neon orange.
Accordingly, the color purities of the respective three primary colors, red, green and blue, should be improved through the optical filter provided for each cell and also the discharge color of neon gas sealed in panels should be removed as far as possible. These are essential subjects for improving color purity and for expanding color reproducibility, as required for display units of plasma display panels.